Micro-electro-mechanical system (MEMS) and apparatus for generating power responsive to mechanical vibration

ABSTRACT

A micro-electro-mechanical system (MEMS) power generator employing a plurality of magnetic masses disposed to oscillate on spring elements in a manner that produces an unusually steep flux gradient at one or more conductive coils, thereby harvesting a substantial portion of the available mechanical energy. The energy from ambient mechanical vibration is harvested to produce electrical power sufficient to power individual electronic elements for a variety of low-cost and high-performance distributed sensor systems for medical, automotive, manufacturing, robotics, and household applications.

STATEMENT OF GOVERNMENT INTEREST

The invention described herein may be manufactured and used by or forthe Government of the United States of America for Governmental purposewithout the payment of any royalties thereon or therefore.

FEDERALLY-SPONSORED RESEARCH AND DEVELOPMENT

This application is assigned to the United States Government and isavailable for licensing for commercial purposes. No license is necessarywhen used for Governmental purposes. Licensing and technical inquiriesshould be directed to the Office of Patent Counsel, Space and NavalWarfare Systems Center, San Diego, Code 20012, San Diego, Calif., 92152;telephone (619)553-3001, facsimile (619)553-3821.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to micro-electro-mechanical systems(MEMS) for energy harvesting and more particularly to an electromagneticMEMS power generator for converting vibrational energy into electricalpower.

2. Description of the Related Art

There is a growing interest in the field of miniature sensors inapplications such as medical implants and embedded sensors in buildings.One of the projected goals for micro-electro-mechanical systems (MEMS)technology is to develop low-cost and high-performance distributedsensor systems for medical, automotive, manufacturing, robotics, andhousehold applications. One area that has received little attention ishow to effectively supply the required electrical power to such sensorelements. Many applications require the sensors to be completelyembedded in a structure with no physical connection to the outsideworld. Ideally, the elements of these distributed systems have their ownintegrated power supplies to reduce problems related to interconnection,electronic noise and control system complexity. Efforts are underway todevelop integrated chemically-based power supplies with MEMS devices.Chemical power supply (battery) technology is well-developed for suchapplications but, where shelf life or replacement accessibility is alimiting factor, chemical power supplies may not be suitable for theapplication. Another approach to supplying power to such systems is toinclude a renewable power supply within the sensor element, therebymaking them self-powered Microsystems.

Renewable power supplies convert energy harvested from an existingenergy source within the environment into electrical energy. Thepreferred source of energy depends on the application. Some possibleenergy sources include optical energy from ambient light such assunlight, thermal energy harvested across a temperature gradient, volumeflow energy harvested across a liquid or gas pressure gradient, andmechanical energy harvested from motion and vibration. Of these sources,light and thermal energy have already been exploited for use inmicro-power supplies. However, there are many applications where thereis an insufficient amount of light or thermal energy such as in medicalimplants. Therefore, practitioners in the art have proposed manydifferent power supplies that generate electricity from ambientmechanical energy. Ambient mechanical vibrations inherent in theenvironment, from the movement of our bodies to the hum of a computer,can provide a constant power density of 10 to 50 μW/cc.

Several practitioners have proposed rudimentary vibration-based powergenerators at the University of Sheffield [C. B. Williams, R. B. Yates,“Analysis of a microelectric generator for microsystems,” 8th Intl.Conf. on Solid-State Sens. & Actutators, Stockholm, Sweden. 25-29 Jun.1995, 87-B4, pp. 369-72] and Massachusetts Institute of Technology[Scott Meninger, Jose Oscar Mur-Miranda, Rajeevan Amirtharajah, AnanthaP. Chandrakasan, and Jeffrey H. Lang, “Vibration-to-Electric EnergyConversion,” IEEE Trans. on VLSI Systems Vol. 9, No. 1, pp. 64-76,February 2001]. for example. Williams et al. describe a theoreticalmodel of an electromagnetic micro-generator for harvesting vibrationalenergy by accumulating the current created by changes in magnetic fluxat a copper coil induced by the vibration of a nearby permanent magnet.Meninger, et al. describe a micro-generator that harvests vibrationalenergy by accumulating the voltage created by vibration-induced changesin a variable capacitor.

Others have recently improved on the earlier efforts. For example, Chinget al. [Neil N. H. Ching, H. Y Wong, Wen J. Li, Philip H. W. Leong, andZhiyu Wen, “A laser-micromachined multi-modal resonating powertransducer for wireless sensing systems,” Sensors and Actuators A:Physical, Vol. 97-98, pp. 685-690, 2002.] describe a micromachinedgenerator with enough power to drive an off-the-shelf circuit. For thiswork, Ching et al. prefer micromachining methods to build theirvibration-induced power generator because the methods afford precisecontrol of the mechanical resonance necessary for generator efficiency,and batch fabricability for low-cost mass production of commerciallyviable generators. Similarly, Williams et al. later describe [C. B.Williams, C. Shearwood, M. A. Harradine, P. H. Mellor, T. S. Birch andR. B. Yates, “Development of an electromagnetic micro-generator,” IEEProc.—Circuits Devices Syst., Vol. 148, No. 6, pp. 337-342, December2001] a simple inertial generator built according to their earliertheoretical analysis that is also fabricated by means of micromachining.Other examples include the laser-micromachined electromagnetic generatordescribed by Li et al. [Wen J. Li, Terry C. H. Ho, Gordon M. H. Chan,Philip H. W. Leong and Hui Yung Wong, “Infrared Signal Transmission by aLaser-Micromachined Vibration-Induced Power Generator,” Proc. 43^(rd)IEEE Midwest Symp. on Circuits and Systems, Lansing Mich., 08-11 Aug.2000, pp 236-9], which provides 2 VDC power sufficient to send 140 mspulse trains every minute when subjected to 250 micron vibrations in the64-120 Hz region.

In U.S. Pat. No. 6,127,812, Ghezzo et al. describe an energy extractorthat includes a capacitor that experiences capacitance and voltagechanges in response to movement of a capacitor plate or of a dielectricmaterial. In one embodiment, a third plate is positioned between firstand second plates to create two capacitors of varying capacitances. Inanother embodiment, one capacitor plate is attached by flexible armswhich permit movement across another capacitor plate. The abovecapacitors can be used singularly or with one or more other capacitorsand are rectified either individually or in a cascaded arrangement forsupplying power to a rechargeable energy source. The above capacitorscan be fabricated on a substrate along with supporting electronics suchas diodes. Ghezzo et al. employ varying capacitance and neither considernor suggest any solution the problem of fabricating an electromagneticmicro-generator.

In U.S. Pat. No. 6,722,206 B2, Takeda describes a force sensing devicehaving an element of magnetic material mounted to a substrate such thatanother magneto-electrical material element is subjected to the magneticfield generated by the magnetic member. A movable member is mounted foroscillation in response to vibration and such oscillation changes themagnetic field experienced by the magneto-electrical material, which inturn changes an electrical property of the magneto-electrical material.Takeda neither considers nor suggests any solution the problem offabricating an electromagnetic micro-generator.

Despite the efforts of several practitioners in the art, there stillexists a need in the art for an electromagnetic micro-generator suitablefor inexpensive fabrication in volume at the MEMS scale that cangenerate power sufficient for operating today's microchips. Theelectromagnetic devices known in the art all generally employ a singlemagnetic mass which oscillates on a spring element to change themagnetic flux at a nearby stationary coil. These devices are therebylimited in power output capacity by the limited mass of the singlemagnet, the limited room for a number of coils in the flux field of thesingle magnet and the limited flux slope available at the coils becauseof the single magnetic pole exposed thereto. These unresolved problemsand deficiencies are clearly felt in the art and are solved by thisinvention in the manner described below.

SUMMARY OF THE INVENTION

This invention solves these problems by introducing for the first time amicro-electro-mechanical system (MEMS) power generator employing aplurality ofmagnetic masses disposed to oscillate on spring elements ina manner that produces an unusually steep flux gradient at one or moreconductive coils, thereby harvesting a substantial portion of theavailable mechanical energy.

It is a purpose of this invention to harvest the energy from ambientmechanical vibration to produce electrical power sufficient to powerindividual electronic elements for a variety of low-cost andhigh-performance distributed sensor systems for medical, automotive,manufacturing, robotics, and household applications

In one aspect, the invention is a monolithic micro-generator including asubstrate having a plurality of integral compliant regions, at least twoferromagnetic masses each coupled to a corresponding one or more of theintegral compliant regions such that at least one of the twoferromagnetic masses moves with respect to the substrate responsive tosubstrate acceleration, each ferromagnetic mass having an inner magneticpole disposed such that the two inner magnetic poles are separated fromone another by a flux gap, a coil coupled to the substrate and disposedwithin the flux gap where it is exposed to a changing magnetic fluxarising from motion of at least one of the two ferromagnetic masses withrespect to the substrate, and conductors coupled to the coil forconducting electrical current flowing in response to the changingmagnetic flux.

In another aspect, the invention is a MEMS power generator including asubstrate having a plurality of integral compliant regions; a pluralityof monolithic micro-generators each having at least two ferromagneticmasses each coupled to a corresponding one or more of the integralcompliant regions such that at least one of the two ferromagnetic massesmoves with respect to the substrate responsive to substrateacceleration, each ferromagnetic mass having an inner magnetic poledisposed such that the two inner magnetic poles are separated from oneanother by a flux gap, and a coil coupled to the substrate and disposedwithin the flux gap where it is exposed to a changing magnetic fluxarising from motion of at least one of the two ferromagnetic masses withrespect to the substrate; and conductors coupled to the plurality ofmicro-generator coils for conducting electrical current flowing inresponse to the magnetic flux changes.

In yet another aspect, the invention is a method for fabricating amonolithic micro-generator including the steps of (a) fabricating aplurality of magnet layer elements by performing the steps of (a.1)preparing a first semiconductor substrate having upper and lowersurfaces, (a.2) masking and etching the first semiconductor uppersubstrate surface to define a plurality of coil layer recesses, (a.3)masking and etching the first semiconductor upper substrate surface todefine a plurality of magnet wells, (a.4) masking and etching the firstsemiconductor upper substrate surface to define a plurality of integralcompliant regions, (a.5) masking and etching the first semiconductorupper substrate surface to define a plurality of bonding posts eachhaving an upper surface, and (a.6) disposing a ferromagnetic mass withineach of a plurality of the magnet wells; (b) fabricating a coil layerelement by performing the steps of (b.1) preparing a secondsemiconductor substrate having upper and lower surfaces, (b.2) maskingand etching the second semiconductor upper substrate surface to define acoil well, (b.3) disposing a conductive coil within the coil well, and(b.4) masking and etching the second semiconductor upper substratesurface to define one or more through holes each disposed to accept abonding post; and (c) bonding the upper bonding post surfaces of a firstmagnet layer element to the corresponding upper bonding post surfaces ofa second magnet layer element with a coil layer element disposed betweenthe upper surfaces of the first and second magnet layer elements suchthat each of the one or more bonding posts passes through acorresponding through hole in the coil layer element.

The foregoing, together with other objects, features and advantages ofthis invention, can be better appreciated with reference to thefollowing specification, claims and the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of this invention, reference is nowmade to the following detailed description of the embodiments asillustrated in the accompanying drawing, in which like referencedesignations represent like features throughout the several views andwherein:

FIG. 1 is a schematic diagram illustrating a damped mass-spring modelrepresentative of the micro-generator system of this invention;

FIG. 2 is a chart illustrating the theoretical relationship between coilvoltage, flux density and relative displacement according to classicalelectromagnetic theory for the model of FIG. 1;

FIG. 3 is a diagram illustrating an edge view of several differentcoil/flux configurations available for use in the micro-generator systemof this invention;

FIG. 4 is a diagram illustrating an edge perspective of an exemplaryembodiment of the micro-generator of this invention;

FIG. 5 is a diagram illustrating an edge perspective of an exemplaryembodiment of the micro-electro-mechanical system (MEMS) power generatorsystem of this invention;

FIG. 6, comprising FIGS. 6( a)-(d), is a diagram illustrating an edgeview of an exemplary magnet layer fabrication process of this invention;

FIG. 7, comprising FIGS. 7( a)-(e), is a diagram illustrating an edgeview of an exemplary magnet layer fabrication process of this invention;

FIG. 8 is a diagram illustrating a facial view of the exemplary magnetlayer embodiments of FIGS. 6 and 7;

FIG. 9, comprising FIGS. 9( a)-(d), is a diagram illustrating an edgeview of an exemplary coil layer fabrication process of this invention;

FIG. 10 is a diagram illustrating a facial view of the exemplary coillayer embodiment of FIG. 9;

FIG. 11, comprising FIGS. 11( a)-(c), is a diagram illustrating an edgeview of a first exemplary micro-generator fabrication process of thisinvention using the magnet layer embodiment of FIG. 6; and

FIG. 12, comprising FIGS. 12( a)-(b), is a diagram illustrating an edgeview of a second exemplary micro-generator fabrication process of thisinvention using the magnet layer embodiment of FIG. 7.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a schematic diagram illustrating a damped mass-spring modelrepresentative of the micro-generator system of this invention. Bothelectrical and mechanical damping must be considered in analyzing andoptimizing the design for particular ambient vibration spectra.Referring to FIG. 1, for time t, a mass m, a spring constant k, anelectrical damping factor b_(e), a mechanical damping factor b_(m), anda displacement function z(t), the power P available from the coilcurrent may be expressed as shown in Eqn. 1:

$\begin{matrix}{P = {{\int_{0}^{v}{F{\mathbb{d}v}}} = {{\int_{0}^{v}{b_{e}\overset{.}{z}{\mathbb{d}v}}} = {{b_{e}{\int_{0}^{v}{v{\mathbb{d}v}}}} = {{\frac{1}{2}b_{e}v^{2}} = {\frac{1}{2}b_{e}{\overset{.}{z}}^{2}}}}}}} & \left\lbrack {{Eqn}.\mspace{14mu} 1} \right\rbrack\end{matrix}$Conservation of energy leads to Eqn. 2:m{umlaut over (z)}+(b _(e) +b _(m))ż+kz=−mÿ  [Eqn. 2]Laplacian transformation and the substitution of variables can be shownto provide the following Eqns. 3-10:

$\begin{matrix}{Z = \frac{{- m}\; s^{2}Y}{{m\; s^{2}} + {\left( {b_{e} + b_{m}} \right)s} + k}} & \left\lbrack {{Eqn}.\mspace{14mu} 3} \right\rbrack\end{matrix}$Let: b_(e)=2mξ_(e)ω_(n)b_(m)=2mξ_(m)ω_(n)  [Eqns 4]where ω_(n) ²=k/m.Thus,

$\begin{matrix}{{\overset{.}{Z}} = {\frac{{- j}\;{\omega\left( \frac{\omega}{\omega_{n}^{2}} \right)}}{{2\left( {\xi_{e} + \xi_{n}} \right)\frac{j\;\omega}{\omega_{n}}} + 1 - \left( \frac{\omega}{\omega_{n}} \right)^{2}}{Y}}} & \left\lbrack {{Eqn}.\mspace{14mu} 5} \right\rbrack\end{matrix}$and

$\begin{matrix}{{\overset{.}{P}} = \frac{m\;\xi_{e}\omega_{n}{\omega^{2}\left( \frac{\omega}{\omega_{n}} \right)}^{3}Y^{2}}{\left\lbrack {\left( {2\left( {\xi_{e} + \xi_{m}} \right)\frac{\omega}{\omega_{n}}} \right) + \left( {1 - \left( \frac{\omega}{\omega_{n}} \right)^{2}} \right)} \right\rbrack^{2}}} & \left\lbrack {{Eqn}.\mspace{14mu} 6} \right\rbrack\end{matrix}$or

$\begin{matrix}{{P} = {\frac{m\;\xi_{e}\omega^{3}Y^{2}}{4\left( {\xi_{e} + \xi_{m}} \right)^{2}} = \frac{m\;\xi_{e}A^{2}}{4\;{\omega\left( {\xi_{e} + \xi_{m}} \right)}^{2}}}} & \left\lbrack {{Eqn}.\mspace{14mu} 7} \right\rbrack\end{matrix}$where A=ω²Y.

This is a non-linear problem and, because of the nonlinear nature of thereaction force from the coil current, the system resonance may beoptimized with reference to Eqn. 7 for a given application without undueexperimentation. In general, the inventors have discovered that a higherelectrical damping b_(e) improves power output performance atfrequencies below the mechanical resonant frequency f_(r)=2πω_(n) of thesystem.

FIG. 2 is a chart illustrating the expected coil voltage, flux densityand relative displacement for various electrical and mechanicalassumptions. The acceleration is assumed to be a constant 1.0 m/sec²over the entire frequency range, B_(max)=1 Tesla, k=1 N/m, velocity=50mm/sec, mass=1 mg, and x=1 mm. The inventors have conducted bothexperimental and theoretical tests and have found that the predictionsdisclosed in FIG. 2 agree well with experimental measurementsimplemented on a larger physical scale.

A macro-scale version of the energy harvesting device was fabricated toverify the expected voltage output per coil. The experimental setupconsisted of a one Tesla magnet measuring one inch in diameter and 3/16inches in thickness. It was attached to a spring with sufficient springforce to result in a displacement of 2.5 mm under accelerations of 1.0m/s² at a frequency of 20 Hz. The number of turns in the coil was variedsequentially from 5 to 40 in increments of 5 and voltage outputmeasurements were made for each configuration. It was observed that thevoltage generated per turn of the coil was very close to the expectedvalue of 1 mV/turn using the simple one-dimensional (I-D) modeldescribed above.

A detailed analysis was performed by modeling the magnetic flux densityin two dimensions and summing the total flux density normal to thesurface of the coil. The input was once again assumed to be a 20 Hzsinusoidal input at 1.0 m/s². At each time step, the velocity,displacement from the coil to the magnet and total magnetic flux densitynormal to the surface were calculated. The results of this detailedanalysis confirmed the simple 1-D calculations and the macro-scaleexperimental observations of 1 mV/turn.

FIG. 3 is a diagram illustrating an edge view of several differentcoil/flux configurations. In FIG. 3, a coil 20 is disposed at a flux gap22 formed by the two magnetic masses 24 and 26. In FIGS. 3( a) and 3(b),a “steep” flux gradient region is formed in flux gap 22 by virtue of thesimilar magnetic poles on each edge of flux gap 22. In FIGS. 3( c) and3(d), a “shallow” flux gradient region is formed in flux gap 22 byvirtue of the dissimilar magnetic poles on each edge of flux gap 22. InFIG. 3( a), coil 20 is disposed in flux gap 22 such that any verticalmotion Z(t) of mass 26 with respect to mass 24 and coil 20 produces arapid change in magnetic flux at coil 20. Similarly, in FIG. 3( b) coil20 is disposed in flux gap 22 such that any synchronous vertical motionZ(t) of both masses 24-26 together with respect to coil 20 produces arapid change in magnetic flux at coil 20. In contrast, in FIG. 3( c)coil 20 is disposed in flux gap 22 such that any vertical motion Z(t) ofmass 26 with respect to mass 24 and coil 20 produces a limited change inmagnetic flux at coil 20. Similarly, in FIG. 3( d) coil 20 is disposedin flux gap 22 such that any synchronous horizontal motion Y(t) of bothmasses 24-26 together with respect to coil 20 produces a limited changein magnetic flux at coil 20. Clearly, the coil/flux configurationsillustrated in FIGS. 3( a) and 3(b) are preferred and, in particular,the configuration in FIG. 3( b) is preferred for implementation of themicro-generator of this invention. Moreover, additional magnetic massesmay also be added and the present masses reorganized to form otheruseful geometric configurations are well-suited for implementation asalternative embodiments of the micro-generator of this invention.

FIG. 4 is a diagram illustrating an edge perspective of an exemplaryembodiment 28 of the micro-generator of this invention. Micro-generator28 includes a coil 30 consisting of a plurality of turns ofelectrically-conductive material coupled to the coil terminals 32 and34. Coil 30 is disposed in the flux gap 36 bounded by the inner surfaces38 and 40 of the magnetic masses 42 and 44, respectively. Inner surfaces38 and 40 are shown as the N-poles of magnetic masses 42 and 44 but maybe either polarity provided that both inner surfaces 38 and 40 have thesame magnetic polarity. Magnetic mass 42 is supported by a plurality ofcompliant elements (springs) exemplified by the compliant element 46.Similarly, magnetic mass 44 is supported by a plurality of compliantelements exemplified by the compliant element 48. The free ends ofcompliant elements 46 and 48 are fixed in any useful manner (not shown)with respect to coil 30, thereby allowing magnetic masses 42 and 44 tomove in the Z(t) direction with respect to coil 30 in response toexternal mechanical vibration.

FIG. 5 is a diagram illustrating an edge perspective of an exemplaryembodiment 50 of the micro-electro-mechanical system (MEMS) powergenerator system of this invention. MEMS power generator 50 includes aplurality of the micro-generators of this invention, exemplified bymicro-generator 28, with the individual coil terminals interconnectedsuch that the electrical power generated by each micro-generator isaggregated at the MEMS power generator terminals 52 and 54. Preferably,the plurality of micro-generators composing MEMS generator 50 arecoupled together for fixed exposure to the same ambient vibration.

FIG. 6, comprising FIGS. 6( a)-(d), is a diagram illustrating an edgeview of an exemplary magnet layer fabrication process of this invention.This process begins as shown in FIG. 6( a) with a semiconductor wafer56. The material may be crystalline silicon or any other usefulsemiconductor material. Although the following discussion is limited tothe preparation of a single magnet layer, practitioners in the art canreadily appreciate that many such magnet layer elements may besimultaneously fabricated on a single semiconductor wafer in a singleprocess and separated from the wafer in a wafer dicing process wellknown in the art. FIG. 6( a) illustrates the results of the first stepin this process, which is the preparation of the upper surface 58 andthe lower surface 60 for processing in the usual fashion by cleaning andpolishing as necessary. FIG. 6( b) illustrates the results of the nextstep of this process, which is the masking and deep reactive ion etching(DRIE) of lower surface 60 to define the magnet well 62. FIG. 6( c)illustrates the results of the next step of this process, which is themasking and DRIE of upper surface 58 to define the coil layer recesses64. FIG. 6( d) illustrates the results of the next two steps of thisprocess, which is the masking and DRIE of upper surface 58 to define theintegral compliant regions 66 and the bonding posts 68, therebycompleting the magnet layer subelement 69 substantially as shown.Bonding posts 68 are also shown in FIG. 8 in a wafer facial view (magnetwell 62 should be demarcated with hidden lines to illustrate theexemplary process of FIG. 6 and in solid lines for the exemplary processof FIG. 7). The final thickness of integral compliant regions 66 isestablished to provide the spring constant necessary for the desiredresonant frequency of the final micro-generator (FIG. 11 below). Theopen region 71 in FIG. 8 is etched away completely to leave magnet well62 coupled only by compliant regions 66. The final step of this magnetlayer fabrication process is the disposition of a ferromagnetic mass 70into magnet well 62 of magnet layer subelement 69 (shown in FIG. 11(c)), which may be accomplished immediately following the completion ofmagnet layer subelement 69 shown in FIG. 6( d) or, as illustratedherein, may be deferred until after the assembly of the micro-generatormagnet layer and coil layer elements (FIG. 11).

FIG. 7, comprising FIGS. 7( a)-(e), is a diagram illustrating an edgeview of an alternative magnet layer fabrication process of thisinvention. This process also begins as shown in FIG. 7( a) withsemiconductor wafer 56. FIG. 7( a) illustrates the results of the firststep in this process, which is the preparation of upper surface 58 andlower surface 60 for processing in the usual fashion by cleaning andpolishing as necessary. FIG. 7( b) illustrates the results of the nextstep of this process, which is the masking and DRIE of upper surface 58to define the coil layer recesses 64. FIG. 7( c) illustrates the resultsof the next step of this process, which is the masking and DRIE of uppersurface 58 to define the magnet well 62. FIG. 7( d) illustrates theresults of the next two steps of this process, which is the masking andDRIE of upper surface 58 to define the integral compliant regions 66 andthe bonding posts 68, which are also shown in FIG. 8 in a wafer facialview (magnet well 62 should be demarcated with hidden lines toillustrate the exemplary process of FIG. 6 and in solid lines for theexemplary process of FIG. 7). The final thickness of integral compliantregions 66 is established to provide the spring constant necessary forthe desired resonant frequency of the final micro-generator (FIG. 12below). The open region 71 in FIG. 8 is etched away completely to leavemagnet well 62 coupled only by compliant regions 66. FIG. 7( e)illustrates the results of the final step of this process, which is thedisposition of ferromagnetic mass 70 into magnet well 62. Ferromagneticmass 70 should include a suitably “hard” ferromagnetic material, forexample, sputtered CoPtCr having a 40 KOe field, and must be disposedwith one magnetic pole bonded to the bottom of magnet well 62 and theother pole exposed at the top of mass 70, thereby completing the magnetlayer element 72 substantially as shown.

FIG. 9, comprising FIGS. 9( a)-(d), is a diagram illustrating an edgeview of an exemplary coil layer fabrication process of this invention.This process begins as shown in FIG. 9( a) with a semiconductor wafer74. The material may be crystalline silicon or any other usefulsemiconductor material. Although the following discussion is limited tothe preparation of a single coil layer, practitioners in the art canreadily appreciate that many such coil elements may be simultaneouslyfabricated on a single semiconductor wafer in a single process andseparated from the wafer in a wafer dicing process well known in theart. FIG. 9( a) illustrates the results of the first step in thisprocess, which is the preparation of the upper surface 76 and the lowersurface 78 for processing in the usual fashion by cleaning and polishingas necessary. FIG. 9( b) illustrates the results of the next step ofthis process, which is the masking and DRIE of upper surface 76 todefine the coil well 80. FIG. 9( c) illustrates the results of the nextstep of this process, which is the disposition of a conductive coil 82within coil well 80. The disposition of coil 82 may be accomplishedusing any of several useful techniques well known in the art, such as,for example, ion deposition of copper or aluminum conductors in a maskedpattern, or by bonding a conductive layer (not shown) to the bottom ofcoil well 80 and masking and etching the conductive layer to create thedesired coil geometry, for example. The coil may, for example include2,500 turns in a radius of 1 mm. FIG. 9( d) illustrates the results ofthe final step of this process, which is the masking and DRIE of eitherupper surface 76 or lower surface 78 to define the bonding post throughholes 84, which are also shown in FIG. 10 in a wafer facial view,thereby completing the coil layer element 86 substantially as shown.FIG. 10 also illustrates the two conductive terminals 88 and 90 disposedto permit electrical connection to coil 82.

FIG. 11, comprising FIGS. 11( a)-(c), is a diagram illustrating an edgeview of the fabrication of a first exemplary embodiment 92 of themicro-generator of this invention, which is shown in FIG. 11( c). FIG.11( a) illustrates the results of the first step in this process, whichis the bonding of a coil layer element 86 to a first magnet layersubelement 69A at the bonding surfaces 94A. FIG. 11( b) illustrates theresults of the second step in this process, which is the bonding of asecond magnet layer subelement 69B to coil layer element 86 at thebonding surfaces 94B and to first magnet layer subelement 69A at thebonding post surfaces 96. Note that sufficient clearance is provided topermit coil 82 to remain mechanically isolated from bonding postsurfaces 96 except for the mechanical coupling provided by compliantregions 66. The final step of this micro-generator fabrication processis the disposition of ferromagnetic masses 70A and 70B into magnet well62 of magnet layer subelements 69A and 69B, respectively, which mayinstead be accomplished immediately following the completion of magnetlayer subelement 69 before beginning the assembly of micro-generator 92.

FIG. 12, comprising FIGS. 12( a)-(b), is a diagram illustrating an edgeview of the fabrication of a second exemplary embodiment 98 of themicro-generator of this invention, which is shown in FIG. 12( b). FIG.12( a) illustrates the results of the first step in this process, whichis the bonding of a coil layer element 86 to a first magnet layerelement 72A at the bonding surface 100A. FIG. 12( b) illustrates theresults of the second step in this process, which is the bonding of asecond magnet layer element 72B to coil layer element 86 at the bondingsurfaces 100B and to first magnet layer element 72A at the bonding postsurfaces 102. Note that sufficient clearance is provided to permit coil82 to remain mechanically isolated from bonding post surfaces 102 exceptfor the mechanical coupling provided by compliant regions 66.

Based on measurements and calculations, the inventors suggest that theMEMS power generator of this invention can provide an output power from10 to 500 mW/cc at an output voltage from 100 mV to 5,000 mV.

Clearly, other embodiments and modifications of this invention may occurreadily to those of ordinary skill in the art in view of theseteachings. Therefore, this invention is to be limited only by thefollowing claims, which include all such embodiments and modificationswhen viewed in conjunction with the above specification and accompanyingdrawing.

It will be understood that many additional changes in the details,materials, steps and arrangement of parts, which have been hereindescribed and illustrated to explain the nature of the invention, may bemade by those skilled in the art within the principal and scope of theinvention as expressed in the appended claims.

1. An energy harvesting apparatus comprising: a substrate having aplurality of integral compliant regions; at least two ferromagneticmasses each coupled to a corresponding one or more of the integralcompliant regions such that at least one of the ferromagnetic massesmoves with respect to the substrate responsive to substrateacceleration, each ferromagnetic mass having an inner magnetic poledisposed such that the inner magnetic poles are separated from oneanother by a flux gap, wherein the magnetic polarity of each innermagnetic pole is similar to the magnetic polarity of the inner magneticpole on the opposing side of the flux gap; a coil coupled to thesubstrate and disposed within the flux gap where it is exposed to achanging magnetic flux arising from motion of at least one of theferromagnetic masses with respect to the substrate; and conductorscoupled to the coil for conducting electrical current flowing inresponse to the changing magnetic flux.
 2. The apparatus of claim 1wherein: the two ferromagnetic masses are rigidly coupled to one anotherand disposed to move synchronously.
 3. The apparatus of claim 2 whereinthe coupled ferromagnetic masses move linearly with respect to thesubstrate responsive to substrate acceleration.
 4. The apparatus ofclaim 1 wherein: each ferromagnetic mass and the corresponding one ormore integral compliant regions form a resonant mass-spring systemhaving a resonant frequency between 10 Hz and 50 Hz.
 5. The apparatus ofclaim 1 further comprising: a plurality of independent coils coupled tothe substrate and disposed within the flux gap where the coils areexposed to the changing magnetic flux.
 6. The apparatus of claim 1wherein: the substrate consists essentially of crystalline silicon. 7.The apparatus of claim 1 wherein: the inner magnetic poles form a steepflux gradient region in the flux gap.
 8. A micro-electro-mechanicalsystem (MEMS) power generator comprising: a substrate having a pluralityof integral compliant regions; at least one monolithic micro-generator,each monolithic micro-generator comprising: at least two ferromagneticmasses each coupled to a corresponding one or more of the integralcompliant regions such that at least one of the ferromagnetic massesmoves with respect to the substrate responsive to substrateacceleration, each ferromagnetic mass having an inner magnetic poledisposed such that the inner magnetic poles of the ferromagnetic massesare of the same magnetic polarity and are separated from one another bya flux gap, and a coil coupled to the substrate and disposed within theflux gap where it is exposed to a changing magnetic flux arising frommotion of at least one of the ferromagnetic masses with respect to thesubstrate; and conductors coupled to each micro-generator coil forconducting electrical current flowing in response to the magnetic fluxchanges.
 9. The MEMS power generator of claim 8 wherein: in one or moreof the monolithic micro-generators, the two ferromagnetic masses arerigidly coupled to one another and disposed to move synchronously. 10.The MEMS power generator of claim 9 wherein the coupled ferromagneticmasses move linearly with respect to the substrate responsive tosubstrate acceleration.
 11. The MEMS power generator of claim 8 wherein:in one or more of the monolithic micro-generators, each ferromagneticmass and the corresponding one or more integral compliant regions form aresonant mass-spring system having a resonant frequency between 10 Hzand 50 Hz.
 12. The MEMS power generator of claim 8 wherein: in one ormore of the monolithic micro-generators, a plurality of independentcoils coupled to the substrate and disposed within the flux gap wherethe coils are exposed to the changing magnetic flux.
 13. The MEMS powergenerator of claim 8 wherein: the substrate consists essentially ofcrystalline silicon.
 14. An energy harvester comprising: a substratehaving a plurality of integral compliant regions; two ferromagneticmasses each coupled to one or more of the integral compliant regionssuch that at least one of the ferromagnetic masses moves linearly withrespect to the substrate responsive to substrate acceleration, eachferromagnetic mass having an inner magnetic pole disposed such that theinner magnetic poles of the ferromagnetic masses are separated from oneanother by a flux gap, wherein the magnetic polarity of each innermagnetic pole is similar to the magnetic polarity of the inner magneticpole on the opposing side of the flux gap and the inner magnetic polesform a steep flux gradient region in the flux gap; a coil coupled to thesubstrate and disposed within the flux gap where it is exposed to achanging magnetic flux arising from motion of the ferromagnetic masseswith respect to the substrate; and conductors coupled to the coil forconducting electrical current flowing in response to the changingmagnetic flux.
 15. The energy harvester of claim 14 wherein the twoferromagnetic masses are rigidly coupled to one another and disposed tomove synchronously.
 16. The energy harvester of claim 14 wherein eachferromagnetic mass and one or more integral compliant regions form aresonant mass-spring system having a resonant frequency between 10 Hzand 50 Hz.
 17. The energy harvester of claim 14 wherein the substrateconsists essentially of crystalline silicon.